In a radio system which operates with time division multiple access (e.g., TDMA), data messages and control messages are transmitted in bursts over certain time slots between a base station and one or more mobile stations. The base station and the mobile station both have a transmitter and a receiver side. The transmitter side includes a speech coder, channel coder and a modulator. The receiver side includes corresponding units, namely a demodulator, channel decoder and speech decoder.
Speech to be transmitted from a mobile station to a base station is speech-coded in the transmission side of the mobile station and is divided into speech frames prior to channel coding and transmission in the form of bursts in accordance with the access method (TDMA) concerned. In such transmission systems, where these techniques are used, the voice signal is first coded into digital data usually frame by frame with a frame rate of 20 ms, for example, which equals 160 samples at 8 kHz sampling rate. Next the digital voice data is channel encoded and then transmitted over the channel. At the receiver side the demodulated data is channel decoded and corrected if the data is corrupted. At last, the received voice data is passed to a speech decoder, which regenerates the speech from the voice data. If the received voice data is erroneous, it will result in distorted output speech.
The methods used to improve the performance of such systems are usually referred to as error concealment algorithms or bad frame masking techniques. In general, an error concealment method manipulates the input voice data to the speech decoder to decrease the effects of the transmission errors in the received data. For these techniques to be effective they are highly dependent on an accurate quality measurement. Actions are taken only if the occurrence of errors is detected. Input to the error concealment algorithm except the voice data is information about the "quality" of the data.
It is well known to introduce a so-called BFI (Bad Frame Indicator) into the channel decoder of the various cellular radio systems such as the Global System for Mobile Communication (GSM) or the American Digital Cellular (ADC) system. This gives an indication in the form of a binary signal to the speech decoder on the receiver side, which denotes whether a frame error has occurred or not.
U.S. patent application Ser. No. 08/079,865, entitled, "A Method and an Arrangement for Frame Detection Quality Estimation in the Receiver of a Radio Communication System" filed on Jun. 23, 1993, and incorporated by reference, discloses a quality estimation method which is an improvement over the prior art BFI indication. The method in the co-pending patent application can be used either in the GSM or ADC systems, but the method is described the context of the GSM system. The method of the co-pending patent application improves the quality estimation when detecting information frames (speech or data), by using the soft information that is available in the receiver signal path in conjunction with a so-called neural net, with the purpose of obtaining an error indication which is better and more accurate than the indication given, for instance, by the aforesaid BFI. Such neural nets are known per se, but are applied in a radio receiver for providing improved quality estimation of received information frames (speech or data) in a simple fashion. The method of the co-pending patent application can also be applied to achieve improved quality estimation of parts of a speech frame, for instance a given block or a part of a given block within a speech frame.
In the North American Digital Cellular System which conforms to the Electronic Industries Association Interim Standard 54 (EIA IS-54), an error concealment algorithm is recommended. The quality measurement used to detect erroneous speech data frames is a CRC flag. If no errors are detected the received voice data frame is passed to the speech decoder. If the CRC flag detects an error in the most protected class la bits the previous speech frame energy and spectrum parameters are repeated and passed to the speech decoder. The remaining decoded bits for the frame are passed to the speech decoder without modification.
In the proposed error concealment algorithm of EIA IS-54 both the detection and the masking technique are based on hard actions. The previous accepted frame is used when the CRC detects an error or the present speech frame is used when no CRC error is detected. However, it is not necessarily true that the most optimal solution is to use (1) the unaffected present frame for a CRC which is declared "good" or (2) the previous frame for a detected CRC error.
The CRC check is a hard decision based on a few bits (most sensitive class la bits) and do not detect errors in other bits. It is also possible that the CRC detects errors that are only in the CRC bits or fails to detect errors even if there are errors in the most sensitive bits. There is also the possibility that another stronger signal is demodulated. If it is demodulated correctly, no CRC error will be detected. A CRC error, if indicated, will in this case indicate a fault in this other, stronger signal.
Since the error concealment technique in EIA IS-54 is a hard action based on the binary decision CRC check, the actions do not reflect the probability of errors in the different parameters. A more accurate indication and masking of parameter errors and differentiation of the action for different parameters are not possible. A softer mixture between the good previous frame and the possibly erroneous present frame parameters is not easily and effectively implemented.
By using a soft error concealment technique the speech quality will be improved. The perceived speech quality is enhanced, if a soft mixture between previous and present parameter sets is used. This type of bad frame masking requires a softer error detection and quality measure. The regenerated speech quality will also be improved, if the amount of masking reflects the probability of error for a whole set of parameters or a single parameter. The general problem is to find a soft masking technique that effectively utilizes a soft quality measure.